Luminous substances are used in a wide range of uses, for example in the field of lighting and plasma displays. The function of the luminous substances consists in transforming or converting light of relatively short wavelength (in particular UV radiation <380 nm) into colored light of longer wavelength. Since the emitted light has less energy than the excitation light, this conversion is also designated as “down conversion”.
In fluorescent lamps, usually, a combination of three luminous substances is used in order to convert the UV radiation emitted by the filling gas during discharging into visible radiation of the colors blue, green and red. By suitable mixing ratios and covering densities on the cladding tube, for example, white light with different color temperature such as cool white (ca. 7000 K) to warm white (ca. 3000 K) can be generated. Similar systems are used in plasma displays.
With the development of UV LEDs and blue LEDs of high power densities, luminous substances play an important role when converting these light sources into white light. Thus, yellow emitting luminous substances are used in combination with blue LEDs in order to generate white light from the mixture of partially transmitted blue light and yellow fluorescence radiation.
since in contrast to fluorescent tubes and plasma displays, the luminous substances in the LEDs are not hermetically encapsulated, their stability against external influences is of increased significance in case of LED uses.
It is known that sulphide-based luminous substances, for example Eu:Srs, are very moisture-sensitive and their conversion efficiency decreases accordingly over time; the color of the respective LEDs thereby shifts successively toward blue. The same applies to the thiogallates such as (Ba, Ca, Sr, Eu)Ga2S4 or the orthosilicates such as (Ba, Ca, Sr, Eu) SiO4.
Besides a limited chemical stability, luminous substances also change under temperature load. With increasing temperature, irreversible damage to the luminous substance can occur, which results in a lower quantum efficiency and thus quasi in failure of the luminous substance. Usually, the temperatures necessary for this are not reached during the operation of an LED; however, when processing the luminous substance into a conversion element (e.g. glazing the luminous substance), it has to be ensured that no temperature-related damages to the luminous substances occur. In addition, luminous substances also show a reversible efficiency change with temperature, namely an increase of efficiency with decreasing temperature and a decrease with increasing temperature. As a result of this so-called thermo-quenching effect, the color of the light emitting diode changes during temperature fluctuations, which can also be caused by the LED itself (e.g. during changes of the power input).
Color changes of the LEDs are an undesirable technical phenomenon and their elimination or minimization thus represents a development objective.
In this connection, CE:Y3Al5O12 (cerium-doped yttrium-aluminum garnet or Ce:YAG) has established itself as a yellow luminous substance for phosphor-converted white LEDs. Ce:YAG is very thermally stable (up to 800° C.) and shows low thermo-quenching.
In connection with blue LEDs which emit in the spectral range of ca. 440-480 nm and with a suitable coverage density, Ce:YAG generates white light, is sufficiently chemically and thermally stable, and has a high quantum efficiency.
The fluorescence maximum of Ce:YAG lies at approximately 550 nm; however, as described for example in the document EP 0936682, by forming mixed crystals, it can be shifted into the range of longer wavelength; for this, for example, replacing Y2O3 with Gd2O3 is suitable.
In this manner it is principally possible to generate color temperatures between 3000 K and 8000 K; however, due to the low quantum efficiency, the achievable efficiencies, mainly in the region of low color temperatures, are insufficient.
Another disadvantage is created in that due to the specific spectral distribution of the “white” light generated in this manner, certain color components in the spectrum can be rendered only to a limited extent, and thus the color rendering index for these systems is usually limited to values below 75.